Which attack is used by malicious hackers to crack the phone system?

John works as a Network Security Administrator for NetPerfect Inc. The manager of the company
has told John that the company’s phone bill has increased drastically. John suspects that the
company’s phone system has been cracked by a malicious hacker. Which attack is used by
malicious hackers to crack the phone system?

A.
Sequence++ attack

B.
Phreaking